Novel Identification of Arsenolipids Using Chemical Derivatizations in

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Novel Identification of Arsenolipids Using Chemical Derivatizations in Conjunction with RP-HPLC-ICPMS/ESMS Kenneth O. Amayo,*,†,‡ Andrea Raab,† Eva M. Krupp,†,§ Helga Gunnlaugsdottir,∥ and Jörg Feldmann*,† †

TESLA (Trace Element Speciation Laboratory) Department of Chemistry, Meston Walk, University of Aberdeen, Aberdeen AB24 3UE, Scotland, U.K. ‡ Department of Chemistry, Ambrose Alli University, Ekpoma, Nigeria § ACES Aberdeen Centre of Environmental Sustainability, St Machar Drive, Aberdeen, AB24 3UU, Scotland, U.K. ∥ Matis, Icelandic Food and Biotech R&D, Iceland S Supporting Information *

ABSTRACT: The identification of molecular structures of an arsenolipid is pivotal for its toxicological assessment and in understanding the arsenic cycling in the environment. However, the analysis of these compounds in a lipid matrix is an ongoing challenge. So far, only a few arsenolipids have been reported, including arsenic fatty acids (AsFAs) and arsenic hydrocarbons (AsHCs). By means of RP-HPLCICPMS/ESMS, we investigated Capelin oil (Mallotus villosus) for possible new species of arsenolipids. Twelve arsenolipids were identified in the fish oil including three AsFAs and seven AsHCs. Among the AsHCs, four that were identified had protonotated molecular masses of 305, 331, 347, and 359 and have not been reported before. In addition, the compounds with molecular formulas C20H44AsO+ and C24H44AsO+ were found in low concentrations and showed chromatographic properties and MS data consistent with cationic trimethylarsenio fatty alcohols. Derivatization by acetylation and thiolation coupled with accurate mass spectrometry was successfully used to establish the occurrence of this new class of arsenolipids as cationic trimethylarsenio fatty alcohols (TMAsFOH).

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reported,5 which now enables the identification of intact arsenolipids. A number of these compounds has been reported in seafood samples such as AsFAs, AsHCs, and arsenosugars− phospholipids using reversed-phase HPLC-ICPMS and ESMS.7,14−18 Taleshi et al.16 reported the first identification of intact arsenolipids in Capelin oil in which arsenic was found to be present as a homologous pair of two saturated AsHCs and one unsaturated AsHC with six double bonds (Figure 1).16,19 The result of that study also indicated the presence of other unknown arsenic compounds. Raber et al.20 used GCMS for their analysis and also established the presence of these compounds in Capelin oil. Further attempts were, however, made to screen the fish oil for other homologous arseniccontaining alkanes, but none was identified.20 In the present study, Capelin oil was investigated for possible new arsenolipid species. A simple procedure involving stepwise solvent partitioning is described for the separation of arsenic compounds from the lipid matrix and analysis by reversed-

he presence of arsenic in high concentrations in seafoods has been known for many years,1,2 and because seafoods are a major source of exposure, they have been widely studied. However, the majority of these studies have reported only the water-soluble compounds. Even though arsenic also occurs in high concentrations as lipid-soluble compounds in marine samples,3−5 our knowledge in this area is limited due to difficulty in isolating them from the lipid matrix and the lack of suitable analytical techniques. Though HPLC-ICPMS has been shown to be a valuable technique for the speciation of water-soluble compounds, the need for the use of the organic mobile phase to separate the lipid fractions and the incompatibility of the organic solvent with ICPMS have limited the application of this technique for the analysis of arsenolipids.5−7 This limitation has in the past necessitated chemical hydrolysis of arsenolipids prior to analysis by HPLC-ICPMS, and the information derived by this method has been used to speculate the structures of original compounds.8−13 However, this approach has produced only limited information,14 and as a result, there has been a growing necessity for analytical methods suitable for the identification of intact arsenolipids. Recently, the modification and application of HPLC-ICPMS for direct analysis using the organic mobile phase was © 2013 American Chemical Society

Received: July 9, 2013 Accepted: August 28, 2013 Published: August 28, 2013 9321

dx.doi.org/10.1021/ac4020935 | Anal. Chem. 2013, 85, 9321−9327

Analytical Chemistry

Article

repeated four times with fresh portions of aqueous methanol. The pooled methanol extract was evaporated to dryness and redissolved in 5 mL of methanol for analysis. Speciation of Arsenolipids by RP-HPLC-HR-ICPMS/ESOrbitrap-MS. The arsenic species were separated using a gradient of 0.1% formic acid in water and 0.1% formic acid in methanol on a reverse-phase column (Agilent Eclipse, XBDC18, 4.8 × 150 mm). The eluent flow was split post column for simultaneous detection with high-resolution ICPMS (Element 2, Thermo Scientific) and ESMS (LTQ Orbitrap Discovery, Thermo Scientific). HR-ICPMS (Bremen, Germany) was used in the organic mode with platinum cones, and optional gas (20% Oxygen in argon) was added to prevent deposition of carbon on the interface cones. Standard DMA (V) was used for external calibration and quantification of the arsenic species, and 74Ge was used as the internal standard to monitor fluctuations in intensities due to instability in the plasma. The instrument operating parameters are shown in Table 1.

Figure 1. Arsenic-containing hydrocarbons previously identified in Capelin oil. Positions of double bonds not determined.

phased HPLC along with ICPMS and ES-Orbitrap-MS. Derivatization of compounds was used to confirm their identity.

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EXPERIMENTAL SECTION Sample. The Capelin oil used in this study was obtained ́ from industrial producers of fish oil and fishmeal (Sildarvinnslan hf (SVN) and Vinnslustodin hf (VSV) in Iceland). The Capelin fish (Mallotus villosus) was caught from Icelandic water in December prior to the production process. Reagents and Chemicals. All chemicals used were of analytical grade except where stated otherwise. Ultrapure water (Elga) was used for sample preparation. Formic acid and sodium arsenite were supplied by Sigma Aldrich (U.K.). Sodium dimethylarsinate (DMAV) was used as a calibration standard for quantification of arsenic species and was obtained from ChemService (U.S.A.). Hexane, hydrogen peroxide (H2O2, 32%), and methanol were obtained from Fisher Scientific. Nitric acid (HNO3, 65%) was obtained from from Fluka (U.K.), and DORM-3 was used as the reference sample for fish protein and was obtained from National Research Council Canada (NRCC). Sample Digestion. Approximately 0.1−0.5 g of sample was weighed precisely and combined with 1 mL of concd HNO3 in a 50 mL Greiner tube, and the mixture was left to stand overnight for predigestion. Then, 2 mL of H2O2 was added, and the samples were subjected to open microwave digestion (Mars-5, CEM, U.K.) using a stepwise heating program: 5 min at 50 °C, 5 min at 75 °C, and 25 min at 95 °C. The digested contents were diluted to 25 mL, and total arsenic was determined by ICPMS. Total Arsenic Analysis. The total arsenic content in fish oil samples and extracts was determined by ICPMS (Agilent 7500c, Japan) after microwave digestion, as described above. 74 Ge was used as the internal standard, and quantification was carried out against standard solutions of sodium arsenite. The possible ArCl+ interference on m/z 75 (As) was checked by measurement of m/z 77 (Se) and m/z 82 (Se) signals. The accuracy of the measurement was assessed by analysis of the certified reference material DORM-3 from the NRCC. The certified and measured values of 6.88 ± 0.30 and 6.79 ± 0.81 μg As/g, respectively, for DORM-3 showed good agreement with the recovery of 99%. Stepwise Extraction. Capelin oil was partitioned between hexane and aqueous methanol using a modification of reported methods.5,16,20 Approximately 17 g of the fish oil was dissolved in 50 mL of hexane, and then 30 mL of aqueous methanol (1:9, water/methanol) was added. After the mixture was shaken, it was allowed to stand overnight. Afterward, the methanol phase was separated, and extraction of the residual hexane phase was

Table 1. HPLC-ICPMS/ESMS Parameters for Arsenolipid Speciation Analysis HPLC column column temperature injection volume buffer A buffer B splitter ratio flow rate gradient

Thermo Scientific Agilent Eclipse, XBD-C18, 4.8 × 150 mm 30 °C 100 μL 0.1% formic acid in water 0.1% formic acid in methanol 1:3 1 mL/min 0−25 min: 0−100%, 5 min: 100% B

ICPMS mode HF(W) nebulizer nebulizer gas (L/min) optional gas plasma gas coolant gas (L/min)

Element 2 (Thermo Scientific) organic mode 1370 microconcentric 0.86 20 mL/min O2 0.89 14.9

ESMS mode spray voltage normalized collision energy

LTQ Orbitrap Discovery (Thermo Scientific) positive 4.5 kV 35%

Acetylation and Thiolation of Arsenolipids. The lipid extract was acetylated by adding 1 mL of extract to 1 mL of acetic anhydride. One milliliter of pyridine was added as the catalyst, and the reaction mixture was stirred. After 11 h, the mixture was evaporated and redissolved in methanol for analysis. Thiolation was carried out by bubbling H2S gas into the extract.

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RESULTS AND DISCUSSION Stepwise Solvent Extraction and Total Arsenic Determination. Measurements of total arsenic were carried out in triplicate, and the results were expressed as mean ± SD. The analysis for total arsenic showed that 4.10 ± 0.13 μg As/g was originally present in the fish oil. Hexane and methanol or methanol-based mixtures such as methanol/chloroform have often been used for the extractions of arsenolipids from marine organisms.7,17 The hexane extract has been shown to 9322



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Figure 2. Reversed-phase HPLC-ICPMS of the methanol extract showing seven arsenic peaks with a close-up in the inset to reveal the minor arsenic peaks at low intensities.

Table 2. Quantification of Arsenic Species by HPLC-ICPMSa

a

As peaks

A

B

C

D

E

F

G

total

concn (μg As/g) % of species

0.12 3.01

0.02 0.50

0.05 1.25

0.005 0.13

0.05 1.25

3.13 78.45

0.63 15.79

3.99 100

Result obtained in μg As/g of oil by measuring peak areas (A−G) against the standard concentration of DMA (V).

Table 3. Molecular Formulas and Accurate Masses of Twelve Arsenolipids in Capelin Oil, Recorded by the Orbitrap and Simultaneously Detected at m/z 75 by the ICPMS peak

compds

exptl MH

calcd MH

Δm (ppm)

B C1 C2 D E F1 F2 F3 F4 F5 G1 G2

C17H36AsO3 C23H38AsO3 C24H38AsO3 C15H34AsO C17H36AsO C17H38AsO C18H40AsO C19H40AsO C23H38AsO C24H40AsO C19H42AsO C20H44AsO

363.1866 437.2031 449.2025 305.1815 331.1969 333.2127 347.2281 359.2281 405.2127 419.2284 361.2439 375.2595

363.1871 437.2027 449.2027 305.1817 331.1973 333.2129 347.2285 359.2285 405.2129 419.2285 361.2441 375.2597

1.38 −1.21 0.45 0.6 1.21 0.60 1.15 1.11 0.49 0.24 0.55 0.53

Figure 4. Proposed methylation and protonation of G2 and G1 in the electrospray, where R = C17H35.

lipids when arsenolipid extract was partitioned between hexane and methanol, whereas 5% of total lipid and about 50% of the arsenic were found in the methanol fraction. The difference in the partition behavior of the matrix in different solvents (hexane and aqueous methanol) was harnessed for purification purposes, which permitted the separation of the arsenic species from the oil matrix in a simple and convenient manner. Following the stepwise extraction of 17 g of oil, the pooled sample was preconcentrated to a final volume of 5 mL containing 13.23 μg As/g of methanol extract, which is equivalent to 3.87 ± 0.27 μg As/g of oil. The extraction efficiency expressed in percentage recovery (94%) was evaluated by comparing the concentration of arsenic extracted in methanol phase (3.87 ± 0.27 μg As/g of oil) with the original concentration in the fish oil (4.10 ± 0.13 μg As/g of oil). Repeated extraction and preconcentration was necessary to enhance the recovery of arsenic compounds, and the methanol extract was analyzed without further purification. Detection and Quantification of Arsenic Species by HPLC-ICPMS. The chromatogram in Figure 2 revealed the presence of at least seven arsenic-containing peaks (A−G) from the reversed-phase HPLC-ICPMS of the fish oil extract.

Figure 3. Section of RP-HPLC chromatograms obtained for the methanol extract of Capelin oil, showing minor and major arsenic peaks from ICPMS at m/z 75 overlaid with ESMS (molecular specific) signals.

accumulate a high concentration of the oil matrix.20 Taleshi et al.14 reported a hexane layer containing over 90% of the total 9323



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Figure 5. ESMS full-scan positive ion mode accurate mass spectrum of compound G2 (C20H44AsO+), M+ = 375 (a), the reaction schemes for acetylation using acetic acid anhydride of compound G2 (b). The acetylated product (C22H46AsO2+) was identified using Orbitrap-MS, as shown in the mass spectrum (c). Similarly, the accurate mass of acetylated F5 (C26H42AsO2+) was monitored by the Orbitrap-MS, which is clearly shown in the mass spectrum (d).

The high-resolving power of the Orbitrap was essential to distinguish the different arsenic species that coeluted, as indicated by the overlay of the ICPMS elemental signal and the molecular specific signals from the ESMS in Figure 3. Separation of the arsenic species by methanol gradient in the reversed-phase column was caused by the degree of hydrophobicity of the compounds. The levels of unsaturation and chain lengths of the arsenolipid compounds also contributed to the chromatographic behavior where different arsenolipids coeluted to form broad peaks in the arsenic trace of the ICPMS. Compounds F5 and G2 were also identified. Although the elemental compositions of F5 and G2 are consistent with AsHCs, their structures have been proposed to be cationic trimethylarsenio fatty alcohols (TMAsFOH), which is consistent with their chromatograhic properties coupled with MS data (more information is provided below). Also, an unknown peak A that eluted with the void volume of the chromatogram was detected. Previously, this peak has been reported as a possible degradation product of arsenolipids.18 The mass spectrum in full scan, positive ion mode between m/z 325 and 365 at the retention time of 23.6−24.6 min showing the accurate masses and elemental compositions of four arsenolipids (E, F1, F2, and F3) is illustrated in Figure 6a.

Quantification of individual peaks (compounds) was achieved by measurement of the peak areas (Table 2). For quantification, the arsenic response factor was monitored and corrected as described previously,7 and the calibration curve prepared in the concentration range of 10−1000 μg As/g showed excellent linearity (R2 > 0.9999). Chromatographic recovery (103%) was evaluated by comparing the quantification result of the extract by HPLC-ICPMS (3.99 μg As/g of oil) with the total concentration of arsenic in the digest determined by ICPMS (3.87 ± 0.27 μg As/g of oil). Identification of Arsenolipids by HPLC-ICPMS/ESOrbitrap-MS. Three known AsFAs (B, C1, and C2) and seven AsHCs (D, E, F1, F2, F3, F4, and G1) were identified based on their MS and MS/MS data. The molecular formulas and accurate masses determined by Orbitrap-MS are shown in Table 3, with Δm = ± 0.24−1.38 ppm. The AsHCs, F1, F4, and G1 with protonated molecular masses of 333, 405, and 361, respectively, constituted the major arsenolipids of over 80% of the total arsenic content in the fish oil; this is in line with previous findings of arsenolipids in Capelin oil.7,16 Compounds D, E, F2, and F3 have not been reported previously and were identified here as AsHCs in low concentrations. The RPHPLC-ICPMS/ES-Orbitrap chromatogram is shown in Figure 3. The elemental arsenic was monitored in parallel by ICPMS. 9324



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Figure 6. ESMS full-scan positive ion mode accurate mass spectrum at m/z 310−360, showing the accurate masses and elemental compositions of four arsenolipids E, F1, F2, and F3 (a), mass spectrum (MS/MS) of F3, protonated molecular ion MH+ = 359 showing some of the major fragment ions (b), and the MS/MS for G2 in (c).

presence of G2 (C20H44AsO+ with Δm = 0.05 ppm), confirming that the compound G2 is not a product of methylation. The occurrence of this new class of arsenolipid was further established using chemical derivatizations by acetylation and thiolation coupled with mass spectrometric analysis. Chemical Derivatization and Identification of New Compounds. Chemical derivatization by acetylation and thiolation provided further evidence for the occurrence of TMAsFOH. The products of these reactions were monitored by ESMS. It is expected that if G2 (C20H44AsO+) was originally present as an arsenic hydrocarbon with a terminal dimethylarsinoyl moiety, the thiolated analogue (C20H44AsS+) will be detected in the electrospray. Similar reactions have been reported in the literature, where the oxygen of the dimethyl arsenoyl moiety was replaced by sulfur to form the thio analogue.21−24 In this case, the mass spectra of the reaction mixture did not show any thiolated compound that corresponds to G2. However, the second experiment involving acetylation of the extract gave the corresponding acetate that was detected in the electrospray, suggesting the presence of one hydroxyl group (alcohol). Acid anhydrides are known to react readily with alcohols, and acetic anhydride, for example, is often

Trimethylarsenio Fatty Alcohols. The compounds G2 (C20H44AsO) and G1 (C19H42AsO) eluted at the same retention time, as shown in Figure 3, and their elemental compositions as generated by the Orbitrap-MS indicated that both compounds might be saturated AsHCs but with a difference in chain length of one CH2 unit. However, it is unlikely that G1 and G2 will coelute if both compounds are to be saturated AsHCs due to difference in their chain length. Since G1 has already been established as a saturated AsHC7,16,17,20 this implies that G2 is structurally different and of higher polarity than the corresponding AsHC of similar molecular weight or chain length. On the basis of this, the structure for G2 was tentatively proposed as a cationic TMAsFOH, as shown in Figure 7c. Though the elemental composition and chromatographic property are consistent with the proposed structure, it is also conceivable that G2 might be the result of methylation of G1 from the methanol of the mobile phase instead of protonation to give C20H44AsO+ in the electrospray-MS or during chromatography, as shown in Figure 4. For the purpose of determining whether or not G2 was a product of methylation, methanol in the mobile phase was replaced by ethanol, and the Orbitrap-MS for the ethanol gradient in Figure S-1 (Supporting Information) revealed the 9325



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elucidation of TMAsFOH species. The AsHCs exhibited unique fragmentation pattern with the ion intensities at m/z 91, 105, and 123 that correspond to OAs+, (CH3)2As+, and (CH3)2AsOH2+, respectively, and indicate the presence of the dimethylarsinoyl moiety in the molecule. The molecular peaks are usually present as base peak in the mass spectra of AsHC, and an example is given for F3 (m/z = 359) in Figure 6b. However, the MS/MS spectrum of G2 shown in Figure 6c did not give enough information for structural identification, with only a few fragments along the As-carbon chain, but it could be deduced from the mass spectrum that the fragmentation pattern is quite different from those observed for AsHCs, in particular the absence of fragment ions indicative of terminal dimethylarsinoyl groups. On the basis of this, the structure for G2 was tentatively proposed as cationic TMAsFOH, as shown in Figure 7c.

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CONCLUSION Identification of the chemical species within seafood is essential in assessing potential toxicity associated with arsenolipids.1,21,24 However, there are still many unknown arsenolipid compounds present in various marine samples due to difficulties in their isolation and identification. In this work, we demonstrated that chemical derivatizations can be used for the identification of novel arsenolipids. The presence of cationic trimethylarsenio lipids such as the fatty alcohols (TMAsFOH), a new class of arsenolipids, was established using the tailored derivatization technique in addition to simultaneous use of elemental and molecular mass spectrometers as online detectors for RPHPLC. Arsenic fatty acids and hydrocarbons with terminal dimethyarsinoyl groups have been identified in seafood, but fatty alcohols with a positively charged terminal trimethylarsonium group have not been reported until now. This could be as the result of their low concentrations in the complex matrix, and the origin of these new compounds is not currently known.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 7. Three AsFAs (a), seven AsHCs (b), and two TMAsFOH (c) identified in fish oil extracted from Capelin. Compounds D, E, F2, F3, F5, and G2 are newly identified .

Notes

The authors declare no competing financial interest.

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used to make derivatives of natural products.25 To verify the presence of the arsenic−fatty alcohol, the hydroxyl group was acetylated to give the corresponding arsenic acetate, C22H46AsO2+ (Figure 5b), which was detected by the Orbitrap-MS. The mass spectrum in Figure 5c clearly showed the presence of this compound with a Δm of −3.11 ppm. F5 exhibits similar properties to G2 and was also investigated. Compounds F5 and F4 shared some similarities. Their elemental compositions are consistent with polyunsaturated AsHC with same number of double bonds. However, F5 coeluted with F4, despite having a longer chain length (Figure 3), and the mass spectrum for acetylated F5 in Figure 5d indicated that it is present as cationic TMAsFOH. Additional information from collision-induced dissociation (CID) tandem mass spectrometry was utilized for structural

ACKNOWLEDGMENTS K.O.A. thanks the Ambrose Alli University in collaboration with the Education Trust Fund of Nigeria for financial support. The authors thank Cornelius Brombach for the art work.

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Novel Identification of Arsenolipids Using Chemical Derivatizations in

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